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Serena Williams talks post U.S. Open

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The world No. 1 beat second seed Victoria Azarenka 7-5 6-7 6-1 in New York

Williams had an operation to remove life-threatening blood clots from her lungs in 2011

The American has now won 17 grand slam singles titles

Two years after losing part of her lung during an operation to remove a life-threatening blood clot, newly-crowned U.S. Open champion Serena Williams insists playing tennis "is just fun now."

The world No. 1's battle-weary opponents might not be having so much fun after Williams successfully defended the Flushing Meadows title she won in 2012 against the same opponent, beating second seed Victoria Azarenka 7-5 6-7 6-1 to secure her fifth singles championship in New York.

It is a far cry from where the now 17-time grand slam singles champion was in July 2010 when she suffered a cut to her right foot and was later diagnosed with a pulmonary embolism which left her on her death bed.

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Since then four grand slams have been won by Williams during a truly dominant 14-month spell.

One more major triumph would see her sit level with legendary compatriots Martina Navratilova and Chris Evert in fourth position on the list of the most prolific female grand slam champions of all time.

"It was the toughest thing I've been through in my life," the 31-year-old told CNN. "One thing kept happening after another, blood clots in both of my lungs, I lost part of my lung, I had to retrain. I don't have two full lungs anymore.

"You go through this stage of thinking, 'Why does all of this keep happening.' It was really 11 months of hell.

"When I'm on that court and I'm facing opponents, I feel like I've faced so many tougher opponents that this is just fun now."

Williams' lung problems only became obvious when she was rushed into hospital after experiencing breathing difficulties en route to an Oscars party in March 2011.

Photos: The French Open: Women's singles final 12 photos

Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Serena Williams of the United States celebrates match point against Maria Sharapova of Russia during their women's singles final match of the French Open at Roland Garros in Paris on Saturday, June 8. Williams won 6-4, 6-4.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Williams celebrates with the Coupe Suzanne Lenglen trophy following her victory.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Sharapova serves to Williams.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Williams plays a forehand against Sharapova.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Sharapova serves to Williams.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Sharapova serves against Williams.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Williams returns a shot to Sharapova.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Sharapova returns to Williams.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Sharapova serves to Williams.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Williams reacts after a point against Sharapova during their match.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Williams returns a shot to Sharapova.

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Photos: The French Open: Women's singles final12 photos

The French Open: Women's singles final – Tennis fans watch the big screen outside Court Suzanne Lenglen in Paris before the match between Williams and Sharapova.

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Inspiration for a generation 12 photos

Inspiration for a generation12 photos

Top award – President Barack Obama awarded Bille Jean King the Presidential Medal of Freedom in the East Room of the White House in 2009.

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Inspiration for a generation12 photos

Rising star – King made her tennis debut back in 1959. Here, competing under her maiden name Moffitt, she plays a forehand volley during her Wimbledon semifinal in 1964.

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Inspiration for a generation12 photos

Inspiration for a generation – King holds aloft the trophy after beating Ann Jones to win the women's singles title at the Wimbledon Lawn Tennis Championships in 1967.

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Inspiration for a generation12 photos

Inspiration for a generation – Billie Jean King with Bobby Riggs before the 'Battle of the Sexes' match in 1973.

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Routed Riggs – The 55-year-old Bobby Riggs cuts a forlorn figure as he slips to defeat against King in the famous "Battle of the Sexes" at Houston in 1973.

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Inspiration for a generation12 photos

Iconic match – Riggs and King embrace after the famous encounter in 1973. King won the match in straight sets.

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Inspiration for a generation12 photos

Film double – Actress Holly Hunter played King in a special TV film based on the Riggs match, which had a $100,000 winner-takes-all prize.

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Inspiration for a generation12 photos

Wightman wonders – King was the senior member of a powerful 1978 U.S. Wightman Cup team which also included Chris Evert and Tracey Austin.

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Inspiration for a generation12 photos

Glittering prizes – King presents Maria Sharapova with a check for $1 million after the Russian superstar won the WTA Championships in 2004 -- beating Serena Wiliams in the final.

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Inspiration for a generation12 photos

Star gathering – King is joined by Lindsay Davenport, Andy Roddick, Elton John, Serena Williams, Anna Kournikova, Tommy Haas and Jan-Michael Gambill at a World Team Tennis charity day.

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Inspiration for a generation12 photos

Captain fantastic – King, in her role as captain of the United States Fed Cup team, gives a pep talk to Monica Seles during a 2000 tie against Belgium.

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Inspiration for a generation12 photos

Happy 40th – WTA founder King is flanked by Sharapova and Williams at a special gathering of former No. 1s to mark the organization's 40th anniversary.

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"Being in the emergency room and being in hospital for all of that time, not knowing if I would ever pick up a racket again, not even caring, just wanting to be healthy," recalled Williams. "That was a tough time for me."

She is indeed healthy again and operating at the peak of her powers at an age when many of her contemporaries are seemingly struggling to last the pace.

Roger Federer has seen his vice-like grip on men's tennis loosen as he heads deeper into his 30s, while Wimbledon champion Marion Bartoli retired last month at the age of 28.